How much support force does a table exert on a book that weighs 15 N when the book is placed on the table?
What if a hand pushes down on the book with a force of 20 N?
What if a rope lifts up on the book with a force of 10 N? (The hand is no longer there.)
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Answer:
The support force exerted by the table on the book is equal to the weight of the book, which in this case is 15N. The table provides a reaction force to the book which is necessary to maintain the book's equilibrium. The table must provide a resistance that is equal and opposite to the weight of the book to ensure it stays in place. So the support force exerted by the table on the book is equal to the weight of the book, which is 15N.
If a hand pushes down on the book with a force of 20N, the support force exerted by the table on the book will also increase to 20N to counter and resist the downward force from the hand. This is necessary to maintain the book's equilibrium. The table must provide enough resistance to counteract the hand's force and maintain the book's equilibrium. So if a hand pushes down on the book with a force of 20N, the support force exerted by the table on the book must increase to match it and equal 20N.
If a rope lifts up on the book with a force of 10N and the hand is no longer pushing down on the book, the support force exerted by the table on the book will decrease to 10N to match the rope's upward force. This is necessary to maintain the book's equilibrium. The table must provide enough resistance to the rope's upward force to counteract it and maintain the book's equilibrium. So since the rope is now the only force acting on the book, the support force from the table must be equal and opposite to it, which means 10N.